Scene of trolley bus blast in Volgograd, Russia that killed at least 10 say after suicide bombing in same city killed at least 17; officials say second blast was also likely terrorist act. (DENIS TYRIN, AP)

Moscow (CNN) -- A suspected mastermind of twin bomb attacks in the Russian city of Volgograd was killed in a police operation in the restive North Caucasus republic of Dagestan, state media reported Wednesday.